The invention relates to crosslinkable protective colloids, processes for the polymerization of ethylenically unsaturated monomers using the crosslinkable protective colloids, and polymers and polymer dispersions obtainable with these, and their use.
The stabilization of polymer dispersions with protective colloids and surfactants is known. An overview of the customary stabilizers is to be found, for example, in G. Schulz "Die Kunststoffe" [Plastics], C. Hanser Verlag, 1964, or in J. C. Jonson "Emulsifiers and Emulsifying Techniques" Noyes Data Corp., Park Ridge, N.J., 1979, and in Ullmann, "Encylkopadia der technischen Chemie" [Enclyclopaedia of Industrial Chemistry], 4th edition, Volume 10, Verlag Weinheim, 1980.
These compounds are distinguished by a hydrophilic and a hydrophobic part in the molecule, the latter serving for anchoring in the polymer resin. It is generally assumed that these surface-active substances are chiefly located at the interface between the polymer particles and the aqueous liquor. These stabilizers thus greatly influence the total surface area of the system and therefore in the end also the profile of properties of a polymer dispersion. The choice of a suitable emulsifying auxiliary thus ultimately determines the desired final properties of a polymer dispersion.
The use of surface-active polymers as protective colloids is also often described in the more recent patent literature. In a departure from the conventional polymeric protective colloids, such as polyvinyl alcohol, cellutose derivatives or gelatin, new synthesis routes are taken here, and water-soluble polymers with ionic groups, so-called polyelectrolytes, are employed as protective colloids.
An unsaturated polyester of maleic or fumaric acid polyethylene glycol ester which is used for stabilizing emulsion polymers is known from DE 4212786 (U.S. Pat No. 5385971). The polymer dispersions prepared with these are distinguished by good stability to frost and electrolytes. DE-A 4304014 (WO-A 94/18249) describes a process for the continuous polymerization of acrylic polymers with a low degree of polymerization in an aqueous medium. The polymers thus obtained are suitable as dispersing similarities for pigments and of binders. However, a disadvantage of these products is a certain thickener action, which is undesirable for many applications.
The stabilization of dispersion with poly-electrolytes is also known from DE 2540468 (GB-A 1551126), in which a good stability in respect of exposure to salts and shear is attributed to the resulting dispersion. WO-A 92/03482 describes oligomeric carboxylic acids as stabilizers for emulsion polymerization of monomers which can be polymerized by free radicals, which are said to be particularly suitable for the preparation of binders for printing inks. Binder dispersions for textile uses are claimed in U.S. Pat No. 5314943. The polymer dispersions described therein are stabilized by means of a protective colloid containing carboxylic acids groups. Fast setting properties are attributed to them.
Aqueous polymer dispersion of crosslinkable polymers are often employed for the preparation of binders which lead to very stable polymer films, for example in the use as adhesives or coating compositions. Crosslinkable polymers with N-methylol-functional commoner units as crosslinker groups are widely used. Such crosslinkable polymers are known, for example, from DE-C 2512589 (U.S. Pat. No. 4044197). However, the strength of the polymer films obtainable with these cannot be increased without problems by increasing the content of N-methylol-functional units in the polymer. It is therefore proposed in the prior art to increase the crosslinking action of such polymers by special preparation processes (EP-B 205862) or to polymerize commoners with a blocked N-methylol function (EP-A 261378).
Against this prior art, there was therefore the object of providing polymers or aqueous polymer dispersions which form polymer films of high mechanical strength and high resistance to water and solvents.
The object has been achieved by providing a water-soluble, crosslinkable polymer with protective colloid properties which has functional groups which are capable of auto-crosslinking, in particular N-methylol groups, incorporated into the molecular chain.
Water-soluble polymers of high NMA content tend to have high molecular weights with a correspondingly high viscosity of the aqueous solution. Water-soluble acrylic compounds, such as acrylic acid or N-methylol-acrylmide, which is customary as a crosslinking agent, in fact tend towards very high degrees of polymerization, which severely impedes widespread use because of the resulting high viscosities. For example, EP-B 94898 (US-A 4736005) describes the preparation of very high molecular weight (MW&lt;1,000,000) terpolymers of 30 to 95% of dimethylacrylamide (DMA), 0.1 to 10% of N-methylolacrylaamide (NMA) and 4 to 50% of acrrylamido-methylpropanesulphonate (AMPS) and their use as rheo-logical additives in crude oil production. Another disadvantage of polyelectrolytes based on water-soluble monomers is that such polyelectrolytes are often incompatible with a polymer dispersion and the aqueous polyelectrolyte solution separates out from the polymer dispersion as a serum.